which is right?

My brother and I are in the middle of a silly argument😂

My cousins status says "I can't do nothing right." I told my brother it makes no sense, seen as it's a double negative and should have read "I can do nothing right" or " I can't do anything right"... My brother said it's right and you can say it any of the 3 ways.

Which way is right??

(btw her status was about how clumsy she was and making light of the situation)

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